12.5 common source amplifiers
DESCRIPTION
12.5 Common Source Amplifiers. v in ?. v o ?. Voltage gain A v =v o / v in ?. Common-Source Amplifiers. C 1 and C 2 are coupling capacitors and C s is the bypass capacitor. The capacitors are intended to have large impedances for the dc signal and very small impedances for the ac signal. - PowerPoint PPT PresentationTRANSCRIPT
12.5 Common Source Amplifiers
vin?
vo?
Voltage gainAv=vo/ vin?
Common-Source Amplifiers
•C1 and C2 are coupling capacitors and Cs is the bypass capacitor. The capacitors are intended to have large impedances for the dc signal and very small impedances for the ac signal.
large is or ,0,0
0,
2
1
CfZc
fZc
fCjZc
Common-Source Amplifiers
•For DC analysis, the capacitors are replaced by open circuits to determine the quiescent operation point (Q point). The transconductance gm for the small-signal equivalent circuit is also determined.
•For AC analysis, the capacitor are replaced by short circuits to determine the ac voltage gain Av=vo/vin.
Coupling capacitors
Bypass capacitors
DC voltage sourcesDC Analysis
The Small-Signal Equivalent Circuit
•In small-signal midband analysis of FET amplifiers, the coupling capacitors, bypass capacitors, and dc voltage sources are replaced by short circuits. •The FET is replaced with its small-signal equivalent circuit. Then, we write circuit equations and derive useful expressions for gains, input impedance, and output impedance.
Coupling capacitors
Bypass capacitors
DC voltage sourcesAC Analysis
Coupling capacitors
Bypass capacitors
DC voltage sources
SMALL-SIGNAL EQUIVALENT CIRCUITS
(12.4)
A more complex equivalent circuit consider drain resistance rd
)()( tvgti gsmd
ddsgsmd rvtvgti /)()(
point
1
QDS
Dd v
ir
Common Source Amplifiers: FET source 端接ground
Common Source Amplifiers:
LDdL RRr
R111
1
Lmo
v Rgv
vA
in
Equivalent load resistance
Input voltage & output voltage
gsin
Lgsmo
vv
Rvgv
Voltage Gain
Common Source Amplifiers:
Input Resistance
Output resistance •disconnect the load,•replace the signal source by the internal resistance,• find the resistance by looking into the output terminals.
21in
inin RRR
i
vR G
dDo rR
R11
1
Example 12.4
Analyze the following circuit. KP=50uA/V2, Vto=2 V, L=10um, W=400um (identical to example 12.2).Assume
Find
•midband voltage gain
•input resistance
•output resistance
•output voltage
dr
ttv mV)2000sin(100)(
Example 12.4
Analyze the following circuit. KP=50uA/V2, Vto=2 V, L=10um, W=400um (identical to example 12.2).
DC Analysis
Fine Q point (see example 12.2)
mAVVKI toGSQDQ 784.0)( 2
Example 12.4
Analyze the following circuit. KP=50uA/V2, Vto=2 V, L=10um, W=400um (identical to example 12.2).
AC Analysis
Fine gm(see Ch 12.4)
mSILWKPVVKg DQtoGSQm 77.1/2)(2
3197111
1
LDdL RRr
R
Equivalent load resistance
dr( )
66.5in
Lmo
v Rgv
vA
Voltage Gain
Example 12.4
Analyze the following circuit. KP=50uA/V2, Vto=2 V, L=10um, W=400um (identical to example 12.2).
Input Resistance
kRRRi
vR G 75021
in
inin
Output Resistance
kRrR
R DdD
o 7.411
1
Input voltage mV)2000sin(23.88)()()( tRR
Rtvtvtv
in
ingsin
Output voltage mV)2000sin(500)()( ttvAtv invo